I have reproduced this by changing something in the setting that'll make max button non-highlighted.
I tried very much to reproduce the same thing doing what you did. I couldn't until I figured out something weird.
Yup, there are settings that won't trigger the issue and I haven't tested them all.
What works at my end are the ones listed in my original reply, any one of the three.
I've also noticed that once a setting inactivated the max button, it won't do it again the second time unless Electrum restart.
The small change was probably as a result of me adjusting the fee rate. Shouldn't the Max button be enough for Electrum to understand that I want to spend all the Bitcoins to the specified Bitcoin address?
The problem here is that you didn't hit the "send" button right after you adjusted the fees.
I believe that's not the issue, it originated in the 'send' tab's "
max" button so, he just have to click "
Max" and then "
Pay..." without doing anything in between that can deactivate the max toggle.